In devices that use a battery as a drive power source, such as electric bicycles, electric motorcycles, and electric cars, a battery pack that houses large-capacity secondary batteries is used. Lithium-ion secondary batteries that are high in both volumetric energy density and mass energy density are suitable as drive-power-source batteries.
Among the known lithium-ion batteries are a columnar battery, which is made by winding up a stacked product in which a positive electrode and a negative electrode are stacked through a separator, and a flat battery, which is a stacked product in which a positive electrode and a negative electrode are stacked through a separator.
Among those batteries, the flat battery is suitable as a power-source battery for a power motor and the like, because the capacity can be easily increased per unit battery by increasing the areas of the positive and negative electrodes or by increasing the number of positive and negative electrodes stacked.
Even if the number of positive and negative electrodes that are stacked as battery elements is increased to increase the capacity per unit battery, in order to minimize an increase in the volume of the battery, the distances from a positive electrode tab and a negative electrode tab, which are taken out from the positive and negative electrodes and are used for conductive connection, to a positive electrode pull-out tab and a negative electrode pull-out tab need to be made shorter. Meanwhile, if the positive electrode tab and the negative electrode tab are being connected to the positive electrode pull-out tab and the negative electrode pull-out tab by the shortest distances, the curvature of the positive and negative electrode tabs gradually increases as the battery elements are located closer to an outer surface of a stacked body. Therefore, if a large force is applied to the positive and negative electrode tabs, the possibility is high that the positive and negative electrode tabs would break.
As a solution to the above problem, for example, Patent Document 1 is proposed.
Incidentally, in the lithium-ion secondary battery, in order to avoid the deposition of metallic lithium in an end portion of the negative electrode at the time of charging, the negative electrode is made larger than the positive electrode, and, in a portion of the negative electrode that faces a portion to which a positive-electrode active material has been applied, a negative-electrode active material needs to be formed.